What you are hearing is an edit of audio from the first moon landing, July 21st, 1969. The first bit is Neil Armstrong’s famous quote, just after he became the first human to set foot upon the moon. The audio then cuts to a transmission from Houston a little later in the mission:

Bruce McCandless (CAPCOM): Roger. The EVA is progressing beautifully. I believe they are setting up the flag now. [BEEP] I guess you’re about the only person around that doesn’t have TV coverage of the scene. [BEEP]

Michael Collins (CMP): That’s all right. I don’t mind a bit.

During that transmission, Bruce McCandless was sitting rather firmly on Earth, serving as Capsule Communicator (aka CAPCOM, the one person who is designated to be point of verbal communication with the spacecraft). Michael Collins, on the other hand, was on the moon with Buzz Aldrin and Neil Armstrong. That’s a long distance over which to communicate via radio waves — a distance long enough to cause a three-second delay.

In fact, there were several issues to overcome in communicating to spacecraft via radio. That’s where those [BEEP]s come into our story.

Those beeps are called Quindar tones. Wikipedia has a decent description of what they do:

For Mission Control (in Houston, Texas) to stay in continuous contact with the astronauts as they traveled to and from the Moon, NASA used several tracking stations around the world, switching from one to the next as the planet turned. Each station had an Apollo Unified S-Band (USB) earth station connected to Houston with dedicated telephone lines. The USB system was full duplex but microphone switching was still needed to block local conversations and background noise from being transmitted. The astronauts needed only push-to-talk or VOX switching, but this was insufficient at Houston because the telephone lines connecting it to the tracking stations were analog and subject to noise and crosstalk when the channel was quiet. This could annoy the astronauts or disturb their sleep.

This meant that the mute switch that controlled the Houston-to-spacecraft audio had to be located at the tracking station transmitter, and the purpose of the Quindar tones was to operate this switch remotely. The same system was used in Project Gemini and were still in use with half duplex UHF Space Shuttle communications for transmitter RF switching.

In short, the Quindar tones work a bit like the button on a walkie-talkie: you push the button to speak, then release it when you are done. The difference is that the button was only on one side (Houston’s), and the button is operated by a pair of tones instead of a thumb.

There are two Quindar tones, one for “on” (a sine wave of 2,525 Hz) and one for “off” (a sine wave of 2,475 Hz). (Sound source: Wikimedia Commons)

The modules that received these tones had a notch filter that removed the tones themselves from the Earth-to-moon transmissions. Thus, we could hear the Quindar tones, but the astronauts could not. (This page on NASA’s website goes into a little more detail about that.)

Quindar tones are named after the manufacturer of the tone generation and detection equipment, Quindar Electronics, Inc. Quindar Electronics is actually still around today, under the name CG Automation, in Springfield, NJ. They currently make monitoring equipment for utility industries. If you want to see what they’ve been up to since the 1960s, there is a detailed history of the company here.

Where do Quindar tones crop up in pop culture? Well, this might not be a direct descendant of the tones, but it is the first thing I thought of:

While Star Trek‘s original TV series (1966) predates the moon landing (1969), the Quindar tones were in use during the Gemini Program, which had flights in 1965 and 1966. My guess is that the United States’ fascination with the space program in the 1960s had something to do with the fact that everything on the Enterprise bridge seems to have some sort of random beeping noise.